Zhen Chen

Photon-Dressed Bloch-Siegert Shift in an Ultrastrongly Coupled Circuit Quantum Electrodynamical System

  1. Shuai-Peng Wang,
  2. Guo-Qiang Zhang,
  3. Yimin Wang,
  4. Zhen Chen,
  5. Tiefu Li,
  6. J. S. Tsai,
  7. Shi-Yao Zhu,
  8. and J. Q. You
A cavity quantum electrodynamical (QED) system beyond the strong-coupling regime is expected to exhibit intriguing quantum phenomena. Here we report a direct measurement of the photon-dressed
qubit transition frequencies up to four photons by harnessing the same type of state transitions in an ultrastrongly coupled circuit-QED system realized by inductively coupling a superconducting flux qubit to a coplanar-waveguide resonator. This demonstrates a convincing observation of the photon-dressed Bloch-Siegert shift in the ultrastrongly coupled quantum system. Moreover, our results show that the photon-dressed Bloch-Siegert shift becomes more pronounced as the photon number increases, which is a characteristic of the quantum Rabi model.
arxiv pdf

Method for identifying electromagnetically induced transparency in a tunable circuit quantum electrodynamics system

  1. Qi-Chun Liu,
  2. Tie-Fu Li,
  3. Xiao-Qing Luo,
  4. Hu Zhao,
  5. Wei Xiong,
  6. Ying-Shan Zhang,
  7. Zhen Chen,
  8. J. S. Liu,
  9. Wei Chen,
  10. Franco Nori,
  11. J. S. Tsai,
  12. and J. Q. You
Electromagnetically induced transparency (EIT) has been realized in atomic systems, but fulfilling the EIT conditions for artificial atoms made from superconducting circuits is a more
difficult task. Here we report an experimental observation of the EIT in a tunable three-dimensional transmon by probing the cavity transmission. To fulfill the EIT conditions, we tune the transmon to adjust its damping rates by utilizing the effect of the cavity on the transmon states. From the experimental observations, we clearly identify the EIT and Autler-Townes splitting (ATS) regimes as well as the transition regime in between. Also, the experimental data demonstrate that the threshold ΩAIC determined by the Akaike information criterion can describe the EIT-ATS transition better than the threshold ΩEIT given by the EIT theory.
arxiv pdf

Multi-photon sideband transitions in an ultrastrongly-coupled circuit quantum electrodynamics system

  1. Zhen Chen,
  2. Yimin Wang,
  3. Tiefu Li,
  4. Lin Tian,
  5. Yueyin Qiu,
  6. Kunihiro Inomata,
  7. Fumiki Yoshihara,
  8. Siyuan Han,
  9. Franco Nori,
  10. J. S. Tsai,
  11. and J. Q. You
Ultrastrong coupling in circuit quantum electrodynamics systems not only provides a platform to study the quantum Rabi model, but it can also facilitate the implementation of quantum
logic operations via high-lying resonator states. In this regime, quantum manifolds with different excitation numbers are intrinsically connected via the counter-rotating interactions, which can result in multi-photon processes. Recent experiments have demonstrated ultrastrong coupling in superconducting qubits electromagnetically coupled to superconducting resonators. Here we report the experimental observation of multiphoton sideband transitions of a superconducting flux qubit coupled to a coplanar waveguide resonator in the ultrastrong coupling regime. With a coupling strength reaching about 10% of the fundamental frequency of the resonator, we obtain clear signatures of higher-order red-sideband transitions and the first-order blue-sideband transition in a transmission spectroscopic measurement. This study advances the understanding of driven ultrastrongly-coupled systems.
arxiv pdf